I agree, stay away from runflats.
Now the BMW Mobility Kit is just a fancy name for a stopleak foam spray, but it gets the job done. I doubt anyway you will have the room to stow a spare-wheel in the trunk, even a space-saver one.
Frankly I wouldn't bother, the foam spray is enough in most circumstances, and if there is a larger puncture in the tire, call Bmw Assistance.